Wireless Local Area Network (WLAN) technology has been developed as the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard. IEEE 802.11a and 802.11b use an unlicensed band at 2.5 GHz or 5 GHz. IEEE 802.11b provides a transfer rate of 11 Mbps and IEEE 802.11a provides a transfer rate of 54 Mbps. IEEE 802.11g applies Orthogonal Frequency Division Multiplexing (OFDM) at 2.4 GHz to provide a transfer rate of 54 Mbps. IEEE 802.11n applies Multiple Input Multiple Output (MIMO)-OFDM to provide a transfer rate of 300 Mbps for four spatial streams. IEEE 802.11n supports a channel bandwidth up to 40 MHz and, in this case, provides a transfer rate of 600 Mbps.
Currently, IEEE 802.11af that specifies operation of an unlicensed device in a TV White Space (TVWS) band is being developed.
A TVWS includes Very High Frequency (VHF) bands (54 to 60, 76 to 88, and 174 to 216 MHz) and Ultra High Frequency (UHF) bands (470 to 698 MHz) allocated for TV broadcast and refers to frequency bands in which use of unlicensed devices is permitted under the condition that it does not hinder communication of licensed devices (a TV and a wireless microphone) operating at the corresponding frequency bands.
All unlicensed devices are permitted to operate in frequency bands of 512 to 608 MHz and 614 to 698 MHz except in a few special cases. However, frequency bands of 54 to 60 MHz, 76 to 88 MHz, 174 to 216 MHz, and 470 to 512 MHz are used only for communication between fixed devices. The fixed devices refer to devices which perform transmission only in a determined location. In the following description, a white space band includes the above-described TVWS but is not limited thereto.
An unlicensed device which desires to use the white space band should provide a protection function for a licensed device. Accordingly, the unlicensed device must confirm whether the licensed device occupies the corresponding band before starting signal transmission in the white space band.
To this end, the unlicensed device must obtain information about an available channel list (i.e. a set of available channels) in a corresponding area by accessing a Geo-location Database (GDB) through the Internet or a dedicated network. The Geo-location Database (GDB) is a database for storing and managing information about licensed devices registered therewith and information about channel use which dynamically varies according to geographic location and channel use time of the licensed device.
A station (STA) can perform a spectrum sensing mechanism. An energy detection scheme, a feature detection scheme, etc. can be used as the spectrum sensing mechanism. For example, it is possible to determine that a channel is being used by a licensed device or an incumbent user when the intensity of a received signal exceeds a predetermined value or a DTV preamble is detected. When it is determined that a channel immediately adjacent to a currently used channel is used by an incumbent user, an STA and an AP need to decrease transmit (Tx) power.
Meanwhile, the STA operating in the white space band may also have mobility. If a specific STA moves from a region of one AP to a region of another AP, association between the specific STA and a legacy AP is severed and association between the specific STA and a new AP is then created in such a manner that the specific STA can be handed over from the legacy CP to the new AP.